A telco can cooperate with a service provider to offer a free subscription to the service provided by the service provider, an upgraded subscription with improved features, or the like, e.g., as part of a campaign to attract new customers or satisfy current customers of the telco.
This implies that the telco customer has to create an account with the service provider (if not already having one) and then login to the telco customer portal and enter the username of the service account. The telco can then contact the service provider and activate the offer for the given username.
This is a well functioning way of providing the offer to a telco customer, who is also a user of the service, but there is a risk that some customers find the procedure too complicated or do not manage to perform the manual steps needed correctly for it to work. Also, if may, for security and integrity reasons, not be desirable that the telco is provided with the username of the service account.